Bones one small animal from the rear (5)
I believe the answer is:
tarsi
'bones' is the definition.
(bones of the foot)
'one small animal from the rear' is the wordplay.
'one' becomes 'i' (Roman numeral).
'small' becomes 's' (abbreviation - e.g. clothes size).
'animal' becomes 'rat' (rat is a kind of animal).
'from the rear' says the letters should be written backwards.
'i'+'s'+'rat'='israt'
'israt' backwards is 'TARSI'.
(Other definitions for tarsi that I've seen before include "Bones of the ankles and heels" , "Ankle bones" , "parts of leg" , "collections of bones?" , "Foot bones" .)